FIG. 1 illustrates a representative Wavelength Division Multiplexed (WDM) or Dense Wavelength Division Multiplexed (DWDM) optical system 2 known in the art. As may be seen in FIG. 1, the system 2 comprises a set of transmitters 4 and a set of receivers 6 interconnected by an optical link 8, which in this illustration is taken to encompass the entire optical path between the transmitters 4 and the receivers 6. Thus the link 8 includes optical fiber patch-cords 10 between each Tx 4 and a MUX 12 which operates to combine optical signals from each of the TXs 4 to generate a WDM or DWDM signal, which is then transmitted through a plurality of fiber spans 14 interconnected by discrete optical devices 16 to a receiver end of the link 8. At the receiver end, a DEMUX 18 separates individual wavelength channels from the inbound WDM or DWDM signal, and supplies each separated wavelength channel to a respective receiver 6 via an optical patch-cord 20.
Typically, each optical component within the link 8 has at least some degree of birefringence. Consequently, the link 8 as a whole also exhibits a link birefringence that is the combined effect of the respective component birefringence of each optical component making up the optical link 8. As is known in the art, the birefringence of some components can vary in time at rates of up to 20 kHz or more. This time varying birefringence can be detected in a receiver 6 as a rapidly time varying Polarization Mode Dispersion (PMD).
Methods are known for compensating PMD in a receiver 6 by coherent detection of the channel signal and post-processing in a high speed Digital Signal Processor. However, in some cases, a lower cost solution would be desirable, even at a cost of somewhat reduced performance.